BalanceHeaderFiat gradually became a complex component as more features were added to Cashtab, i.e. support for multiple fiat currencies. As it became more complicated, this additions were added everywhere the component appeared (Send.js, Wallet.js, Tokens.js).
Better practice is to keep this type of component in one place and then import it where it is used. That way, you only have to edit things in one place.
This diff does that. While I was at it, I couldn't resist also displaying the fiat currency as a locale string (i.e. with commas for americans, spaces for others) -- since this is also a task and some currencies generate very large numbers.